I prefer mortars. Wiz towers, to me, are rarely my best defender, maybe 1 out of 20 battles they are the deciding factor. Usually, by the time they start firing, attackers already start hitting them, so they can only fire a few times before dying. Mortars, however, have often been the difference between winning and losing.
That being said, i would do the low wiz towers first, then the low mortars, then high wt, high mortar. The reason is that since mortars are better at defense, i need wts to be higher level for the same protection while the mortars are upgrading, but a lower mortar would do a better job protecting during wt upgrades.
In a nutshell, lower mortars can protect better during wt upgrades, but you need higher wts during mortar upgrades. Either way though, you get beat up during any mortar upgrade.
